diff --git a/dlx.py b/dlx.py
--- a/dlx.py
+++ b/dlx.py
@@ -14,7 +14,7 @@ class Cell:
 
 class Column(Cell):
 	def __init__(self,name,*args,**kwargs):
-		super().__init__(*args,**kwargs)
+		super().__init__(*args,up=self,down=self,**kwargs)
 		self.name=name
 		self.size=0
 
@@ -45,10 +45,11 @@ class Column(Cell):
 		self.left.right=self
 
 
-class Header:
-	def __init__(self,left=None,right=None):
-		self.left=left
-		self.right=right
+class Header(Column):
+	def __init__(self):
+		super().__init__("head")
+		self.left=self
+		self.right=self
 		self._res=[]
 
 	def search(self,k=0):